Numerous magnesium alloys of greatly varying composition and usability are known. For purposes of the present invention, the term magnesium alloy is primarily understood to include a group of alloys which, in addition to magnesium as the main component, contain additives (typically up to approximately 10%) of aluminum, manganese, zinc, copper, nickel, cerium misch metal and other rare earth metals, silver, zirconium, silicon, etc. Magnesium alloys are divided into magnesium wrought alloys (typically based on Mg—Mn, Mg—Al—Zn) and magnesium cast alloys; the magnesium cast alloys are subdivided by sand casting, chill casting, and die casting or according to alloy components. Magnesium alloys may be processed according to most known metallurgical primary forming and reshaping methods.
The alloy additives decisively determine the properties of the metallic material. It is known, for example, that aluminum contents greater than approximately 10 weight-percent result in brittleness of the alloys. Zinc and especially zirconium increase the toughness, while manganese improves the corrosion resistance. Beryllium additives of a few parts per million significantly reduce the tendency of the molten metal to oxidize, but are undesirable because of their toxicity. Rare earth metals and thorium increase the heat resistance. The melting point of the alloys is typically between 590° and 650° C.
The main areas of use of magnesium alloys are aviation, mechanical engineering of all types, optical devices, electrical technology, electronics, means of transportation, office machines and domestic appliances, and, in general, areas in which strength and rigidity at the lowest possible weight are important and low manufacturing costs in large productions are required. Magnesium alloys are receiving increasing significance in engine construction for motor vehicles. A special application relates to the use of biodegradable magnesium alloys in medical technology, in particular for vascular and orthopedic implants.
A limitation of known magnesium alloys particularly comprises the ductility of the material, which is inadequate for specific processing methods and intended uses. One approach for improvement may be in reducing the grain size of the metallic microstructure (fining). Fining comprises all metallurgical measures which result in a small grain size of the alloy. In general, this requires an increase of the number of nuclei in the melt during solidification or in the solid body due to finely dispersed precipitates. Fining has an advantageous effect on the mechanical properties, in particular the ductility of the alloy. Very small grain sizes have been achieved until now only in the small preparative scale in the field of magnesium alloys, for example, by ECAP/ECAE methods (ECAP is equal channel angular pressing; ECAE is equal channel angular extrusion). The cited methods may not be implemented in large scale, however; up to this point, only small volumes (a few cm3) of extremely fine-grained alloys have been produced, predominantly using the technical magnesium alloy AZ31. Findings of a generally valid nature about the alloy components required for fining or even their proportion in magnesium alloys have only been available in inadequate form up to this point.
Therefore, there is a persistent need for magnesium alloys which permit fining even using typical large-scale methods. Furthermore, there is a need for a magnesium alloy whose grain size is reduced for improved ductility in relation to typical alloys. Moreover, there is a need for a preparation method for a fine-grained magnesium alloy which may be technically implemented in larger scale. Finally, in regard to ecological aspects and also a use of the alloys in medical technology, it is necessary to select the alloy components under toxicological and/or biocompatible aspects; the biocompatible aspects, in particular, while avoiding the aluminum present in many magnesium alloys.